Maintenance / Read Only Mode

### Is your feature request related to a problem?

After updating, I recognised some major errors, but I couldn't roll back because of the changes to the database structures and the newly written data. It would be useful to be able to activate a read-only mode before updating, so that we can back up the database, update it, and then roll it back if necessary.

### Describe the solution you'd like

A maintenance mode that disables all write processes without failing.

### Describe alternatives that you've considered

The process involves shutting down completely, backing up the database, performing the update and rolling back if necessary. However, this approach incurs more downtime for simple logins.

### Additional context

_No response_

The issue names no files, tests, or entry points, so the first step is to identify where write processes and login handling are implemented. Done means a maintenance mode can be enabled before an update, prevent writes without failing, and still allow simple logins while the database is backed up or rolled back.

Written by the indexing model from the issue text.